This invention relates generally to an electrophotographic printing machine, and more particularly concerns an optical system having a curved screen for modulating the light image of an original document.
A typical electrophotographic printing machine exposes a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive member are discharged recording thereon an electrostatic latent image corresponding to the original document. A development system moves a developer mix of carrier granules and toner particles into contact with the latent image recorded on the photoconductive member. Toner particles are attracted electrostatically from the carrier granules to the latent image. In this manner, a powder image is formed on the photoconductive member. Thereafter, the powder image is transferred to a sheet of support material. After transfer, the sheet of support material passes through a fusing device which permanently affixes the toner powder image thereto.
Multi-color electrophotographic printing employs this basic concept. However, in multi-color electrophotographic printing, each cycle is for a discrete color contained in the original document. Thus, multi-color printing requires the light image to be filtered so as to record an electrostatic latent image on the photoconductive member corresponding to a single color of the original document. This single color electrostatic latent image is developed with toner particles of a color complementary to the color of the filtered light image. Thereafter, the toner powder image is transferred to a sheet of support material. This process is repeated for successively differently colored light images. Each toner powder image is transferred, in superimposed registration with the prior toner powder image, onto the sheet of support material. In this way, a multi-layered toner powder image is formed which corresponds to the colors of the original document. Thereafter, this multi-layered toner powder image is permanently affixed to the sheet of support material forming a permanent color copy of the original document.
In most electrophotographic printing machines, tone gradations are difficult to form. This problem may be obviated by the utilization of screening method. Generally, a screening technique produces the effect of tone gradations by variations in the diameter of the half-tone dots or the width of the half-tone lines comprising the toner powder image created by the screen. In the highlight zones or regions of low optical density, the dots or lines are small increasing in size through the intermediate shades until they merge together in the shadow region. At the highlight end of the tone scale, there will be complete whiteness while at the shadow end nearly solid blackness. The foregoing is described in U.S. Pat. No. 2,598,732 issued to Walkup in 1952. Other patents exemplifying various screening techniques are U.S. Pat. No. 3,535,036 issued to Starkweather in 1970; U.S. Pat. No. 3,121,010 issued to Johnson et al. in 1964; U.S. Pat. No. 3,493,381 issued to Maurer in 1970; U.S. Pat. No. 3,776,633 issued to Frosch in 1973; and U.S. Pat. No. 3,809,555 issued to Marley in 1974. Recently filed applications describing different screening techniques are copending application Ser. No. 511,976 filed in 1974 and copending application Ser. No. 507,169 filed in 1974. In addition, copending application Ser. No. 556,027, filed in 1975, and 566,873, filed in 1975, also relate to different types of curved screens employed in electrophotographic printing machines.
It is well known that the illumination of an image point is in proportion to the cos.sup.4 of the angle between the illumination point and the image point. It can, therefore, be seen that the illumination on a photoconductive surface will fall off quite rapidly as the angle increases. Various techniques have been devised to compensate for this affect. Typically, a sheet of opaque material having a butterfly slit formed therein is employed. The area of the slit is inversely proportional to the illumination profile. In an exposure system of this type, the original document is positioned on a flat transparent platen. The scan lamps and lens move across the original document in synchronism with the rotation of the photoconductive drum. In this way, successive incremental areas of the original document are scanned forming a flowing light image which is projected through the slit. A well known characteristic of such slit exposure systems, i.e. wherein the original document is positioned on a flat platen and the light image passes through the slit onto a curved photoconductive member, is image smearing. Image smearing occurs even if the scan and drum velocity are perfectly synchronized. The loci of exposure points on the drum corresponding to a single point of the original document are defined by the intersection of a plane and a cylinder. The plane is defined by a point (the lens) and a line (normal) to the drum axis and containing any image point on the photoconductive drum. During a slit scan, the image point does not remain stationary on the drum, but rather suffers both lateral and longitudinal translations. Such image motion causes loss of resolution. If a screen member having a plurality of substantially equally spaced opaque lines were placed near the photoconductive surface wherein the lines were aligned parallel to the drum circumference, the lateral image motion would smear the modulation produced by the screen member. In extreme cases, the modulation could be destroyed near the edges of the drum, where smearing is a maximum. For typical drum radii, lens focal lengths, and slit widths, this lateral smearing is highly significant.
Accordingly, it is a primary object of the present invention to improve the optical system so that modulations produced by a screen will not be smeared.